1 files changed, 48 insertions, 0 deletions
diff --git a/src/SMAPI.Web/Framework/RewriteRules/ConditionalRewriteSubdomainRule.cs b/src/SMAPI.Web/Framework/RewriteRules/ConditionalRewriteSubdomainRule.cs
new file mode 100644
index 00000000..920632ab
--- /dev/null
+++ b/src/SMAPI.Web/Framework/RewriteRules/ConditionalRewriteSubdomainRule.cs
@@ -0,0 +1,48 @@
+using System;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Rewrite;
+
+namespace StardewModdingAPI.Web.Framework.RewriteRules
+{
+    /// <summary>Rewrite requests to prepend the subdomain portion (if any) to the path.</summary>
+    /// <remarks>Derived from <a href="https://stackoverflow.com/a/44526747/262123" />.</remarks>
+    internal class ConditionalRewriteSubdomainRule : IRule
+    {
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>A predicate which indicates when the rule should be applied.</summary>
+        private readonly Func<HttpRequest, bool> ShouldRewrite;
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="shouldRewrite">A predicate which indicates when the rule should be applied.</param>
+        public ConditionalRewriteSubdomainRule(Func<HttpRequest, bool> shouldRewrite = null)
+        {
+            this.ShouldRewrite = shouldRewrite ?? (req => true);
+        }
+
+        /// <summary>Applies the rule. Implementations of ApplyRule should set the value for <see cref="RewriteContext.Result" /> (defaults to RuleResult.ContinueRules).</summary>
+        /// <param name="context">The rewrite context.</param>
+        public void ApplyRule(RewriteContext context)
+        {
+            HttpRequest request = context.HttpContext.Request;
+
+            // check condition
+            if (!this.ShouldRewrite(request))
+                return;
+
+            // get host parts
+            string host = request.Host.Host;
+            string[] parts = host.Split('.');
+            if (parts.Length < 2)
+                return;
+
+            // prepend to path
+            request.Path = $"/{parts[0]}{request.Path}";
+        }
+    }
+}